Electron temperature diagnostic line ratios as a function of O/H (left panels) and ([O iii]λ5007 + [O ii]λ3727)/Hβ (right panels) for our models (colored circles). From top to bottom, the diagnostics are [O iii]λ4363/λ5007 and [N ii]λ5755/λ6584. Black diamonds represent the T_e-based sample of H ii regions used by Marino et al. (2013). The dashed lines correspond to the grid of models computed by Dopita et al. (2013). The color bar follows Q_{0 / 1}, the softness of the ionizing radiation.